To be commercially viable, photovoltaic cells must generate electricity at a price competitive with fossil fuels. Photovoltaic cells must therefore be fabricated inexpensively using low cost materials and exhibit moderate-to-high efficiency in the conversion of sunlight to electricity. Additionally, the methods used to synthesize the required materials and the device fabrication method must be scalable.
The high cost of crystalline silicon wafers (the industry standard photovoltaic absorber) has led industry to look at cheaper materials to make solar cells. Semiconductors having the formula CuInSe2 or Cu(In,Ga)Se2 (a.k.a. CIGS) materials are strong light absorbers, with absorption coefficients on the order of 105 cm−1. Consequently, the active layer in a solar cell incorporating CIGS materials can be as small as a few microns thick. Thicker layers are required in silicon-based solar cells because silicon is a relatively poor absorber of light. Moreover, the single crystal silicon wafers currently used are expensive to produce because the process involves fabricating and then accurately slicing high-purity, single-crystal silicon ingots.
Recent effort has focused on producing high efficiency solar cells incorporating a thin film of CIGS semiconductor material. Binary chalcogenide nano-powders including copper selenide, indium selenide, and gallium selenide have been proposed as starting materials for CIGS material. The band gaps of CuInS2 (1.5 eV) and CuInSe2 (1.1 eV) are well matched to the solar spectrum, predisposing them to high conversion efficiency. By 2010, thin-film solar cell efficiencies of up to 20.3% had been achieved for Cu(In,Ga)Se2 materials by researchers at the Centre for Solar Energy and Hydrogen Research in Germany (ZSW).
Absorber layers for CIGS photovoltaic devices can be fabricated using a variety of methods, which generally involve expensive vapour phase or evaporation techniques. Alternatively, nanoparticles can be printed and then melted or fused together to form a thin film, such that the nanoparticles coalesce to form large grains. One such method utilises metal oxide nanoparticles, which are then reduced using H2, after which the resulting film is selenised, usually with H2Se. The costly selenisation step and the use of toxic H2Se can be avoided by incorporating a selenium source into the nanoparticles.
There are a number of techniques currently used to prepare copper selenide nanoparticles. Nanoparticles can be produced using colloidal methods, solvothermal methods, sonochemical methods, surfactant-assisted methods and ball milling of bulk copper selenide produced using a solid state synthetic method. Microbially-mediated routes to the synthesis of CuSe nanoparticles have also been proposed.
Colloidal methods typically involve high temperature (>250° C.) syntheses, such as hot injection, to form nanoparticles capped with trioctylphosphine oxide (TOPO) or amines. Hot injection relies on the injection of small volumes of precursors into a large volume of solvent at elevated temperature. The high temperature breaks down the precursors, initiating nucleation of the nanoparticles. For example, Cu2Se nanoparticles have been made by hot injection of TOP/Cu and TOP/Se into a solution of TOPO and octylphosphonic acid, after which the temperature of the reaction mixture is lowered to support nanoparticle growth over a particular period of time before quenching with a suitable organic solvent.
Solvothermal methods have been studied for the synthesis of copper selenide nanoparticles. However, particle size distribution and solubility are usually very poor. In a typical solvothermal synthesis, copper selenide is formed from the reaction of a copper salt with elemental selenium in an autoclave filled with a gas, such as ammonia.
Sonochemical synthesis typically involves ultrasonic irradiation of a copper salt with a selenium source in the presence of an organic solvent and/or water. The resulting nanoparticles are typically on the order of tens of nanometers to 1 μm in diameter.
Surfactant-assisted pathways for nanoparticle synthesis have been explored for their high reaction yields, shape-controlled nanoparticle formation, and economic and environmental advantages of synthesis in water. Copper selenide nanoparticles have been synthesised via a reaction between copper acetate and sodium selenite in the presence of aqueous hydrazine and an aqueous cationic Gemini surfactant (a surfactant incorporating two surfactant moieties linked by a spacer).
Bulk copper selenide can be formed by solid-state reactions and then milled into nanoparticles. For example, Cu3Se2 can be formed by contacting α-CuSe with α-Cu2Se at high pressure over an extended period of time. α-CuSe can be prepared by heating copper with selenium and α-Cu2Se prepared in a similar manner but by heating to a higher temperature.
Microbially-mediated routes to the synthesis of CuSe nanoparticles have been proposed which employ a source of copper ions and a selenium source in a bacterial culture that can reduce Se to Se2−. Nanoparticle size is typically controlled by the reaction time, which can range from minutes at elevated temperatures to three weeks at lower temperatures. The reaction time to achieve a particular result is influenced by the type of bacteria used, which in turn influences the pH at which the reaction must be carried out. There are therefore a number of factors which must be taken into account when seeking to use such methods to produce high quality copper selenide nanoparticles of a particular size.
To ensure a competitive cost for the manufacture of photovoltaic devices using copper selenide nanoparticles, device fabrication should be relatively cheap. Such techniques include, for example, printing or spraying processes. Existing methods for synthesising copper selenide nanoparticles, as described above, are unfavourable for processing into thin films on a commercial scale as they do not encompass the desirable features of a scalable reaction to generate the required type of nanoparticles having a low melting point, narrow size distribution and a volatile capping ligand. For instance, hot injection techniques produce materials in very low yields and are not easily scaled commercially. Other techniques, such as solvothermal or sonochemical syntheses, do not allow tight control over the physical properties of the nanomaterials. Solid-state reactions form bulk material, which must subsequently be subjected to high-energy ball milling at high temperatures to be broken down into nanoparticles.
Furthermore, current methods for fabricating solar cells are based on traditional vacuum-based deposition processes that require high temperatures and long reaction times to sinter the absorber materials. These methods have several drawbacks, including the need to use expensive equipment to generate the high-vacuum required and substrates that can withstand high temperatures. Further disadvantages are the limited availability of high purity reagents, the reliance upon deposition approaches that are limited to line-of-sight and limited-area sources, which tend to result in poor surface coverage, and, in the case of absorber materials made from more than one element, poor elemental ratio control.
Solar cells that can be printed on flexible substrates represent an attractive, cost-efficient alternative to conventional, vacuum-deposited solar cells because the materials can be deposited using non-vacuum, solution-processable printing technologies. To meet the growing demand for low-cost solar cells employing flexible substrates, there is a need for a simple, low-temperature technique for fabricating high-quality, homogeneous copper selenide nanoparticles that can be dispersed readily into aqueous or organic media to provide economically viable methods for fabricating copper selenide-based devices using solution-processable nanoparticle deposition techniques.